Water Wall Tubes Research Articles

Experimental investigation on flow instabilities of ultra-supercritical water in parallel channels

An experimental study on the flow instabilities of ultra-supercritical water in the water wall tubes of an ultra-supercritical circulating fluidized bed (CFB) boiler was conducted. The material of the tubes is 1Cr18Ni9Ti. The scope of experimental parameters is as follow: pressure from 23 to 30 MPa, mass flow rate from 0.05 to 0.2 kg·s−1; inlet water temperature from 200 to 390 °C, inlet pressure drop coefficient from 0 to 5.5, and heat flux from 0 to 500 kW·m−2. Variations of mass flow rate and pressure in the parallel channels were recorded with the occurrence of flow instability, and oscillation curves of the wall temperature were obtained. The starting points and factors that affect various types of oscillations were analyzed to derive a stability map. Results show that pressure and mass flow rate display a reverse phase pulsation during oscillation. With the increase in heat flux, three regions of oscillations, namely, region1, region2 and region3, appeared. The oscillations in region1 and region2 are observed for the first time under ultra-supercritical pressure. Among them, oscillations in region1 and region2 are system oscillations, and they have long periods and large amplitudes. Oscillation in region3 belongs to density wave oscillation (DWO), which is the intertube oscillation and has a short period and small amplitude. Increases in system pressure, inlet mass flow rate, inlet pressure drop coefficient or decreasing the inlet water temperature are conducive to system stability.

Coupled Heat Transfer Simulation of the Spiral Water Wall in a Double Reheat Ultra-supercritical Boiler

This paper presented a coupled heat transfer model combining the combustion in the furnace and the ultra-supercritical (USC) heat transfer in the water wall tubes. The thermal analysis of the spiral water wall in a 1000 MW double reheat USC boiler was conducted by the coupled heat transfer simulations. The simulation results show that there are two peak heat flux regions on each wall of spiral water wall, where the primary combustion zone and burnt-out zone locate respectively. In the full load condition, the maximal heat flux of the primary combustion zone is close to 500 kW/m2, which is higher than that in the conventional single reheat USC boilers. The heat flux along the furnace width presents a parabolic shape that the values in the furnace center are much higher than that in the corner regions. The distribution of water wall temperature has a perfect accordance with the heat flux distribution of the parabolic shape curves, which can illustrate the distribution of water wall temperature is mainly determined by heat flux on the water wall. The maximal water wall temperature occurs at the middle width of furnace wall and approaches 530°C, which can be allowed by the metal material of water wall tube 12Cr1MoVG. In the primary combustion zone, the wall temperatures in half load are almost close to the values in 75% load condition, caused by the heat transfer deterioration of the subcritical pressure fluid under the high heat flux condition. The simulation results in this study are beneficial to the better design and operational optimization for the double reheat USC boilers.

Study on the effect of flame offset on water wall tube temperature in 600°C and 700°C ultra-supercritical boiler

ABSTRACTThe distribution of water wall temperature in the ultra-supercritical (USC) boilers was obtained by establishing a coupled heat transfer model. The reliability of this model has been validated through the comparisons of simulated and measured water wall temperatures along different dimensions in the furnace of reference USC boiler. Then, the effect of flame offset on water wall tube temperature in 600°C and 700°C USC boilers was investigated by importing the flame offset variables into the coupled model. The water wall temperature distribution is significantly influenced by flame offset, and both the fluctuation and growth rate of temperature are increased with the ascent of flame offset distance, especially on the furnace walls that flame deviates toward. The radiative and convective heat flux to water walls is strengthened simultaneously during the flame offset process, resulting in the local overheating of water wall. In the 600°C USC boilers, when the distance of flame offset exceeds 5 m, multiple peak distribution of wall temperatures appears, which can increase the burst risk of water wall tubes because of shear stress inside the tube material. The maximal distance of flame offset should be limited to 3 m to avoid the tube burst accidents. In the 700°C USC unit, the variation tendency of water wall temperature is resemble with that in 600°C USC unit, but the fluctuation of wall temperature is larger. As the flame offset distance approaches 3 m, the maximal water wall temperature reaches 595°C, which greatly exceeds the material allowable temperature in the 600°C USC unit. The material of water wall tubes with allowable temperature of 605°C is recommended for the 700°C USC unit. Based on the thermal security of metal material, the maximal distance of flame offset should be yielded to 2 m.

Experimental and theoretical analysis on the safety and efficiency of an ultra-supercritical pulverized coal-fired boiler with low mass flux vertical water wall

The heat transfer safety of low mass flux vertical water wall suitable for a high efficiency and wide-load ultra-supercritical coal-fired boiler is investigated experimentally. The pressure drop characteristics of supercritical water are discussed. The net thermal efficiency of an ultra-supercritical pulverized coal-fired power plant is analyzed theoretically by considering the power consumption of the feed water pump. The experimental results demonstrate the test water wall tube exhibits good heat transfer and low pressure drop characteristics at low mass flux and in the normal heat flux range. However, the self-compensation characteristic between parallel tubes can be destroyed at high test heat flux when the test pressure is near the critical value. In this case, the metal temperature and pressure drop of the water wall tube increase fast and abnormally. This phenomenon indicates the distributions of in-tube mass flux and furnace heat flux no longer match, which threatens the heat transfer safety of the water wall. Corresponding analysis demonstrates that on the premise of ensuring the heat transfer safety of the water wall, adopting the low mass flux technology can improve net thermal efficiency of the power plant.

용접부 비파괴 검사의 신뢰성 비교 : 화력 발전소의 보일러 수냉벽 배관 사례연구

용접부 비파괴 검사의 신뢰성 비교 : 화력 발전소의 보일러 수냉벽 배관 사례연구

Failure evaluation of SA 210C riffle water wall tubes in 70 MW CFBC boiler

Abstract Boiler tubes used as water walls made up of ferritic steels is having some finite life, because of prolonged exposure in the furnace at elevated temperature, stress and aggressive environment, tube failure taking place. Now a day's premature failure of the boiler tube mainly in water walls is one of the very common phenomena in the thermal power plants. The present investigation was done on the as-received ASTM SA 210C failed boiler tube steel used as water walls in the Circulating Fluidized Bed Combustion (CFBC) coal fired thermal power plant. An attempt has been made to understand the cause, because identifying correct failure mechanism often helps to perform meaningful life assessment and also to prevent the future Boiler Tube Failure (BTF). The as-received of failed tube along with the parent (unused) tube from the water wall raiser panel were selected to study for mechanical and metallurgical properties. Tensile test and micro hardness examinations were carried out on both the parent tube and failed tube. Visual examination reveals “fish-mouth” appearance of the as-received failed tube because of short-term overheating. The micro structure of the parent metal has the conventional structure of ferrite (white constituent) and pearlite (dark constituent), whereas for the as- received failed tube reveals Widmanstatten ferrite. A detailed structure - property relationship has been made by using the combined techniques of Optical Microscopy (OM), SEM/EDAX and XRD. Tensile fractography depicts the presence of micro-voids coalescence in the fibrous network shows ductile mode failure in the as-received failed boiler tubes.

Failure Analysis of the Corroded Water Wall Tube in a 50MW Thermal Power Plant

AbstractHigh temperature corrosion of the water wall tube in a 50 MW thermal power plant was investigated which caused several boiler accidents. X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS) were used to observe the cross-sectional morphology of the tube and analyze the oxide scales. Results show that the boiler water and the coal quality did not meet the requirements. High temperature corrosion of water wall tubes was attributed to the using of coal which had a higher ash content and lower received lower heating value. Higher dissolved oxygen and incrustation in the boiler water caused serious corrosion at the inner surface of water wall tube, which led to the possibility of decarburization and degradation of the steel. Suitable coal blending and stability of the thermal load were the effective means to prevent the high temperature corrosion of the tube.

Failure Analysis of Boiler Water Wall Tube for Power Generation in District Heating

A corrosion failure of materials, often exposed to the corrosive environment in the district heating facility, has a critical influence on structural integrity. Thus, it is very important to investigate the cause of the failure for the establishment of effective prevention methodology. In this study, the failure analysis was performed in the case that a water wall tube for a boiler in district heating was fractured along the axial direction of the tube that is SA-210 Grade A1, which is frequently used for the boiler. The feed water temperature was about 220°C and the operating pressure was about 10 MPa. For the visual inspection of the failed part, the wall thinning and failure were observed in the location where the heat was directly reached. Sodium and iron oxides were only found in the inner surface of the tube, as confirmed by x-ray diffraction, and it indicates that hydrogen is generated by the mechanism of the caustic corrosion, resulting in hydrogen embrittlement near the inner surface. The microstructural changes along the direction of radial and hoop were intensively examined. A number of microvoids were formed along the grain boundary, leading to some microcracks in the location inside the tube. As a result, it is concluded that an unstable magnetite film at elevated temperature under tensile residual stresses developed along the hoop direction in the interior of the tube undergoes the caustic corrosion, and microvoids or microcracks along the grain boundary formed due to hydrogen embrittlement cause a final fracture of the tube.

A hybrid simulation of a 600 MW supercritical circulating fluidized bed boiler system

A hybrid dynamic model for a 600 MW supercritical circulating fluidized bed (CFB) boiler is presented. The combustion system includes combustion chamber, cyclone separator, standpipe, and ash cooler. The steam-water system mainly contains water wall, external heat exchanger, superheater, reheater, and economizer. The ‘core-annulus cell model’ is employed to simulate complex physical and chemical processes in the boiler. A series of classical empirical models have been chosen to model main physical and chemical processes. The water-steam system is modelled by means of modular modelling method, a new user-defined model for external heat exchanger is established and validated. The combustion system is coupled with the water-steam system via heat exchange in water wall tubes. The static state calculation is taken to compare with the design values indicating the model has reached a rather high precision. Dynamic simulations under three typical transient conditions are conducted which are 5% fuel decrease, 5% secondary air decrease and 5% supply water decrease, respectively. The results demonstrate that the hybrid model is capable of performing good characteristics of CFB. The modelling method has important theoretical and engineering values for researchers, and the hyrbid model can be very helpful to optimize operating and design parameters to get a better performance of large-scale commercial CFB.

Development of 25%Cr SMAW welding materials – development of high Cr ferritic overlay welding materials for recovery boiler

ABSTRACTIn place of the 18%Cr overlay welding material, which has been used for furnace water wall tubes of recovery boilers, a 25%Cr overlay welding material with good corrosion resistance has been newly developed. In consideration of dilution with a carbon steel tube, the Cr content of the new welding materials are designed to be more than 20%Cr. And in order to fine the grains of the weld metal, some elements, such as Nb, Al and Ti, are added to these welding materials. Mainly two types of welding materials, which have Austenite + Martensite + Ferrite of materials for the three-phase and fully ferritic materials, are examined. As results of weldability tests and bending properties, the fully ferritic welding materials are superior to the three-phase materials, respectively. On the point of the corrosion test, which are exposed into some smelt ashes obtained from the recovery boilers, the corrosion rate of the full ferritic material was confirmed to be the about half of that of 18%Cr overlay welding material. The 25%Cr overlay welding material was found to exhibit excellent corrosion resistance. This study introduces the development of 25%Cr ferritic welding material and methods for reliable overlay welding and examines the weldability, the bend property and the corrosion resistance of the 25%Cr overlay welded metal on tube.

Effect of transverse rib on heat transfer between circulating fluidized bed and membrane fins of water wall membrane tubes

Overall heat transfer between circulating fluidized beds (CFB) and the ribbed membrane fins of a water wall tube was investigated experimentally in a cold model circulating fluidized bed riser. The riser had a cross sectional area of 0.1 × 0.2 m2 and the height from the distributor plate to the riser exit was 3.9 m. Three isosceles triangular cross section prisms having a height to base width ratio (e/B) of 0.54 corresponding to the height to riser diameter ratios (e/D) of 0.019, 0.038 and 0.06, respectively were used as ribs. The ribs were installed transversely on the fins with three rib spacings (S) of 0.4 m, 0.5 mand 0.6 m. Local sand having an average diameter of 315 μm was used as the bed material. It was found that the heat transfer coefficients and the cluster solid fractions of the ribbed membrane fins were higher than those of the membrane fin, and varied with the rib spacing and the rib height to diameter ratio in the same manner. In addition, cluster contact time of the ribbed membrane fins was shorter than that for the membrane fin without ribs.

Novel online simulation-ready models of conjugate heat transfer in combustion chamber waterwall tubes of supercritical power boilers

This paper presents two models of fluid heating in waterwall tubes of supercritical steam boilers. The models are named 1D/2D and 1D/3D. The models are formulated and discussed in detail with an industrial application example. For the 1D/2D model, a two-dimensional (2D) transient heat conduction equation is solved for the tube wall with the fin domain, while one-dimensional (1D) mass, momentum, and energy equations are solved for the fluid domain. The 1D/3D model considers a three-dimensional tube wall domain (3D) and one-dimensional fluid domain. At the fluid-solid interface, a conjugate heat transfer model is applied. The model is based on convective flux between the fluid and solid domains. Nonlinear governing balance equations of mass, momentum, and energy for fluid are solved using the forward time backward space (FTBS) scheme. The proposed models allow incorporation of the effect of heat flux nonuniformities along the waterwall tube and on its outer circumference. Transient simulations are carried out to determine the temperature histories for both the fluid and the tube wall in the selected cross sections. The computations are performed using the finite volume method formulation. The results obtained from the 1D/2D model are nearly the same as those from the 1D/3D model; however, the computation time is more than five times shorter. Therefore, an efficient 1D/2D model can be used in power unit simulators.

Evolution of Microstructure During Short-term Overheating Failure of a Boiler Water Wall Tube Made of Carbon Steel

This paper highlights the evolution of different microstructures during short-term overheating failure of a boiler water wall tube made of SA210 Grade C steel. Short-term overheating failure of the boiler tube caused fish mouth rupture with thin lips, along the longitudinal direction of the tube with extensive tube bulging. Microstructure examination revealed elongation of grains at the fracture signifying plastic deformation due to hoop stress becoming equal to the flow stress of the tube at the elevated temperature. Overheating caused degeneration of the initial ferrite–pearlite microstructure. Spheroidization and coalescence of carbides occurred in the ferrite matrix from the initial ferrite–pearlite microstructure of the boiler tube near the ruptured region. Moreover, the formation of bainite microstructure at the fracture fish mouth opening indicates heating above the eutectoid temperature (lower critical temperature) of the tube material.

An experimental study on the hydrodynamic performance of the water-wall system of a 600 MW supercritical CFB boiler

The temperature distribution on the backside of the water-wall tubes in the first 600 MW CFB boiler operating at 60%, 80% and 100% of the boiler Maximum Continuous Rating (MCR) loads was experimentally determined and the thermal-hydrodynamic performance of water-wall system based on the mass and energy equations using the measured temperature was calculated. The results showed that the temperature of the water-wall tubes increased as the furnace height increased, showing a positive response characteristic in the water-wall system. However, the temperature of the water-wall tubes in the upper furnace was obviously declined at the places where the superheaters and cyclone separator inlet-channels were arranged. This suggests that the arrangement of the heat-transfer surfaces in the upper furnace was significantly influenced the temperature distribution of the water-wall tubes. Meanwhile, the calculated total pressure-drop and mass flux in the water-wall system increased as the boiler’s load raised, showing an energy-efficiency for the vertical once-through boiler. In addition, the metal temperature was calculated to check the working condition of the water-wall tubes and the results showed the highest metal temperature of 426.3 °C of the water-wall tubes was found at the tip of the fin at 100% MCR. This temperature was far below the permissible temperature of the metal, implying that a safe working condition of the metal at three loads was guaranteed. Nonetheless, analysis into the heat-transfer characteristics of the water in the water-wall reflects that the water temperature in the water-wall at 80% MCR load was adjacent to the boundary of the large specific-heat region, implying that heat-transfer deterioration was more likely to occur at such condition. Therefore, the metal temperature of the water-wall tubes operating at 80% MCR load requires the most concern and monitor to avoid overheating of the water-wall tubes.

Method for planning the chemical cleaning interval of water wall tubes in thermal power plant boilers based on estimated creep

Method for planning the chemical cleaning interval of water wall tubes in thermal power plant boilers based on estimated creep

Thermal flow analysis of vertical combustion chamber waterwall tubes operation

In this paper will be presented an algorithm that allows to determine the temperature distribution in a vertical pipe through which is flowing the medium with high temperature and pressure. The method of pipe division into control volumes presented in paper allows to determine the temperature distribution on the pipe cross-section and to determine changes in its value at the pipe height. The issue will be solved by taking into account the variability of the parameters of the material from which the pipe is made and the parameters of the fluid, depending on the temperature. The applied algorithm allows to determine the change in the value of individual parameters in time, due to which it is a useful tool for analysis of the operation of the system in the initial stage of its work, as well as with variable parameters of flow.

Review of the literature discussing the problem of uneven thermal stresses in the furnace chamber

Uneven thermal stresses occurring in the walls of the furnace chamber in high power boilers are the reason for the occurrence of significant thermal stresses. The occurrence of excessive stress leads to serious damage to the waterwall tubes, which result in the necessity of more frequent repairs of the boiler. Therefore, finding a way to reduce stress values is one of the key challenges in improving productivity and increase the service life of boilers.In this paper will be presented the review of the literature concerning the problem of uneven temperature distribution on the surface of the combustion chamber walls. There will also be presented studies that discuss the methods of modifying the furnace chamber to reduce the values of the stresses occurring in it.

Experimental Study on the Heat-Transfer Characteristics of a 600 MW Supercritical Circulating Fluidized Bed Boiler

Heat-transfer characteristics such as the distribution of the local heat-flux and the water-side and fire-side heat-transfer coefficients of the water-wall at the dilute phase region of a 600 MW supercritical circulating fluidized bed (CFB) boiler operating at boiler loads of 60%, 80%, and 100%, were experimentally investigated. The temperature of the water-wall at furnace heights of 9 to 55 m was in situ measured using K-type thermocouples, and the water-side heat-transfer coefficient, the distribution of local heat-flux, and the fire-side heat-transfer coefficient between the furnace and water-wall were calculated based on the measured temperature result. The results showed the water-side heat-transfer coefficient was raised from 8600 W·m–2·K–1 to 31000 W·m–2·K–1 as the furnace height increased. Moreover, the water-side heat-transfer coefficient was large enough to ensure the heat-transfer deterioration would not take place in the water-wall tubes. Meanwhile, it was revealed that the heat-flux was highe...

Ultrasonic evaluation of the combined effect of corrosion and overheating in grade 20 steel water-wall boiler tubes

Ultrasonic evaluation of the combined effect of corrosion and overheating in grade 20 steel water-wall boiler tubes

Heat transfer characteristics of spiral water wall tube in a 1000 MW ultra-supercritical boiler with wide operating load mode

Nowadays, heavy peak-shaving task requires ultra-supercritical boilers having fast and flexible response-characteristic. Wide operating loads and quick load-change mode ask for better flow and heat transfer characteristics of the water wall. Under the above backgrounds, heat transfer to water flowing upward in an inclined smooth bare tube was experimentally investigated under the operating conditions of an ultra-supercritical boiler. The experimental results show that the metal temperature of the tested water wall tube is safe from 30% to 90% BMCR. The heat transfer coefficient of single-phase water at bottom side is 1–2 times bigger than that at the top. The Fr number put forward by Kefer et al. can give a convincing prediction on the effect of flow stratification in the tested pressure range 12–18 MPa. The calculated steam quality difference with the Kefer et al. correlation is an average-value prediction on axial length of the transitional region. At supercritical pressures, four widely used heat transfer correlations are evaluated with the experimental data. The comparison results demonstrate that Swenson et al. correlation gives the best prediction on heat transfer. The effect of flow stratification is evaluated by the modified correlation of Petukhov and Popov, which demonstrates that the effect of flow stratification can be neglected in the high enthalpy region.